Currently, many motor vehicles are equipped with a VDC system such as an ESP in order to promote driving safety. The purpose of a VDC system is to maintain the lateral stability of the motor vehicle and thereby prevent a driver from plowing, skidding, or fishtailing during challenging driving conditions. Most VDC systems accomplish this by extending the functionality of the anti-lock braking system (ABS) via a control strategy that provides modulated braking at each individual wheel to induce a yaw moment and/or to adjust the lateral acceleration at each wheel. Some newer systems also use active steering to modulate the driver's control input at the steering wheel which alleviates effects of driver overreaction (or underreaction) during the occurrence of disturbing yaw moments, and also during normal driving. Active steering operates via an additional actuator coupled to the steering system that reacts immediately by introducing a signal to a steering control signal in situations when unexpected yaw moments occur.
The VDC system typically determines current driving conditions using detected parameters and then compares the determined conditions with driver inputs. The VDC uses this information to correct the course of the vehicle in critical situations. Several directly measured parameters are provided as inputs to the system including wheel speed (vwh), steering angle (δ), vehicle yaw rate ({dot over (ψ)}), lateral acceleration (ay), and brake pressure (Pbr), among other measured dynamic parameters. From these inputs several additional dynamics parameters are derived and calculated and then compared to nominal values, i.e., desired lateral dynamics derived from the driver's input. These variables may include the yaw rate and the side slip angle (β) among other vehicle dynamics parameters. Whenever a discrepancy between a variable determined from actual driving conditions and a nominal value is detected, individual wheel slip actuators, such as hydraulic brake valves are actuated to compensate for the discrepancy by modifying the vehicle dynamics, which generally brings the driven vehicle back toward the desired course (other types of actuators may also be used to enhance vehicle stability). For example, when a vehicle oversteers, or fishtails, the outside wheel or wheels may be braked to induce a yaw moment that would counteract the discrepancy. When understeering, or plowing, the rear inside wheel may be braked to achieve the opposite effect. Other combinations of wheels may be activated depending on the version of the VDC system, and the driving situation.
Since the VDC system is a complex, integrated system that relies on the integrity of each of the actuator components to provide flexibility and safety in a wide range of dangerous driving situations as they occur in real time, if any of the actuators was to fail, the VDC typically fails to function entirely, and is automatically switched off. There is as of yet no alternative control system that may be activated in the case of such failure that can compensate for the loss of function in one or more of the VDC actuators. This is a substantial functional disadvantage for any VDC system, and it is a particularly major difficulty for those VDC systems that include electromechanical brakes (EMB), which are expected to replace hydraulic brakes in certain motor vehicles. As these brakes do not work on a ‘purely’ mechanical principle, a mechanical backup cannot be applied if there is a failure in one or more of these brakes. Therefore, systems that employ these brakes require a high degree of fault tolerance and must fail operational.
It would therefore be beneficial to provide a fault-tolerant VDC system that can remain active despite the failure of one or more VDC actuator components, and that has the capability to compensate for such failure and maintain vehicle safety during dangerous driving conditions.